Primary cilia organize signaling pathways such as vision, olfaction and the Hedgehog developmental pathway. The movements of signaling receptors into, inside and out of cilium are critical for the correct regulation of these pathways, yet our understanding of the basic mechanisms governing signaling receptor trafficking through cilia remains fragmentary. The major goal of this proposal is to study the BBSome, a protein complex that ferries signaling receptors into, inside and out of cilia. The emphasis in this fundin period will be on investigating the turnaround point during which anterograde trains become reconfigured into retrograde trains. While past studies have provided circumstantial evidence for the existence of IFT train turnaround at the tip of cilia, molecular mechanisms are lacking for thi critical step that commits molecules to be removed from cilia. The removal of signaling receptors from cilia may serve to desensitize cilia to specific stimuli and the ciliary export of signaling intermediates such as the Hedgehog effector and transcription Gli3 is required for them to reach their nuclear target. Yet, how molecules are exported from cilia remains elusive. Our preliminary data suggest a testable molecular model for the turnaround point and the assembly of retrograde BBSome/IFT trains. We will test this model using biochemistry and innovative live cell photokinetics assay.